some filters require an oil to be put on/in them in order for them to work (k&n is an example) and some don't, they are dry filters. If you over oil the oil-type filter, excess oil can be blown/sucked through and contaminate the afm causing the car to run poorly/cause engine damage.

on a side note, a few magazines have tested a variety of cone/pod filters and found that the cheaper (non-oiled) ones are actually better at flowing and filtering.

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